Since the interior of a picture tube is maintained in a high-vacuum state, the outer periphery of the panel is clamped with a steel band for preventing concave deformation and/or implosion of the front panel and for preventing scattering of the panel glass at the time of implosion. A soft-magnetic, high-strength plated steel sheet of a thickness of about 0.8-2.0 mm is used for the picture tube band. At the time of clamping, the picture tube band is processed into a prescribed shape, thermally expanded by heating to around 450-550° C., fitted on the periphery of the panel portion, and immediately subjected to rapid cooling to implement the shrinkage fitting method to obtain a strong clamping force by the firm fastening of the band. This clamping force precisely corrects the shape of the panel face that was concavely deformed owing to the high-vacuum inside the tube. In addition, the soft magnetic property of the band enables it to serve as a “magnetic shielding material against geomagnetism” that prevents invasion of geomagnetism into the interior of the picture tube. The picture tube band material is therefore required to have high strength and high permeability in a weak DC magnetic field on the order of geomagnetism. In particular, the high-strength property is preferably such as that a yield stress of 300 N/mm2 or greater can be obtained with good stability.
Generally speaking, increasing steel strength and increasing steel permeability are incompatible objectives. For example, although effective methods for enhancing the strength of steel sheet include precipitation strengthening by addition of Ti, Nb or the like, strengthening by ferrite crystal grain refinement, dislocation strengthening by imparting working strain and the like, all of these strengthening measures lower permeability. Various picture tube band steels have been developed heretofore with the aim of achieving these incompatible properties. For example, those taught in the following Patent Documents are known:    Patent Document 1 JP. Hei-10-208670A    Patent Document 2 JP. Hei-10-214578A    Patent Document 3 JP. Hei-11-140601A    Patent Document 4 JP. Hei-11-293397A    Patent Document 5 JP. 2000-290759A    Patent Document 6 JP. 2001-040417A    Patent Document 7 JP. 2001-040418A    Patent Document 8 JP. 2001-040419A    Patent Document 9 JP. 2001-040420A
Patent Documents 1 and 2 teach methods of producing a picture tube band using a so-called “silicon steel sheet,” namely, a cold-rolled steel sheet added with not less than 1% Si and having a C content of not greater than 0.005%. However, the material property required for geomagnetic shielding property improvement is permeability in a weak DC magnetic field and there is no need for the low iron loss in an alternating magnetic field that characterizes silicon steel sheet. Moreover, addition of a large amount of Si to a steel after it has once been reduced to a very low carbon content of C≦0.005% increases cost and also markedly degrades the toughness and ductility of the steel to lower productivity owing to increased susceptibility to cracking during hot rolling and cold rolling. In addition, the so-called temper color that tends to arise during annealing owing to oxidation of Si in the surface layer degrades the plating adherence property.
Patent Document 3 teaches utilization of a Ti-added steel. However, the high recrystallization temperature of a Ti-added steel increases production cost. Moreover, permeability is degraded owing to the fact that fine precipitated carbonitride directly prevents migration of magnetic domain walls and that the ferrite crystal grain diameter is reduced.
Patent Document 4 teaches a steel whose strength is increased by P addition and positive utilization of strain produced by temper rolling and whose low magnetic field property is enhanced by controlling the balance between the crystal grain diameter and the temper rolling. Patent Document 5 teaches improvement of magnetic properties and steel strengthening that is based on carrying out addition of Si and Mn. Patent Documents 6 to 9 teach utilization of age-hardening by solid solution C for strength enhancement and simultaneous achievement of high strength and high permeability without need for extreme carbon reduction or heavy Si addition, by controlling cementite precipitation morphology/size and ferrite crystal grain diameter. In studies conducted by the inventors, however, it was found that these disclosed methods do not necessarily make it possible to realize high yield stress of 300 N/mm2 or higher with good stability.